3 Phase single phase dual voltage motor Star Delta

[Bills Elm]

I dunno if this is the right place for this question but it was a toss up? So here goes....

Can anyone tell me, if I'm using a dual voltage motor can I just change it from delta to star or "vice versa" (I can't remember which way it is) and run it off a 240 volt supply or do I have to use an inverter to run it off 240v? and if so what is the point of a dual voltage motor?

I hope this makes sense and I hope it's in the right forum if I don't get any replies I'll change forums!

Bill

 

[CHJ]

I assume you are talking about a three phase motor.

If so it is normally run Star Connected on 415 volt from standard UK 3 wire supplies.

If you change the links in the motor to run it on Delta connected 240 volt you will need an electronic invertor to convert the UK 240 volt single phase domestic supply to 240 volt three phase for the motor.

The alternate is to use an electronic invertor to convert 240 volt single phase to 415 volt 3 phase and leave the motor alone.

 

[Bills Elm]

Yes it's a three phase motor,

So either way I need an inverter to run it off a 240v single phase supply grrr!

It may actually work out cheaper to change the motor to a single phase one.

Thanks Chas

Bill

 

[graduate_owner]

Hi Bill,
there are definite advantages in keeping your 3-phase motor:
1. you don't need mess around with another motor - shaft size, mounting issues, physical size of motor etc
2. you will be able to alter the motor speed, from almost zero RPM (but perhaps with loss of torque, depending on the inverter) to full speed. plus you can set parameters such as gradual start/stop instead of immediate full speed - kinder on belts etc
3. you will be able to reverse the rotation easily.

Whether advantages 1 and 3 are worth having depends on what the motor is running. On a table saw, probably not, but on a lathe then definitely.
You will be able to use the inverter on other 3-phase kit that you may come across.

Regarding the star / delta issue - the motor must be 415 / 240 volt (some are only 415v -( I had to take one of these to a motor rewinding firm and they picked the windings apart and brought out the extra 3 terminals needed.) otherwise make sure you get an inverter that will produce 415v. These are dearer than the 240v ones.

I use 3-phase now on my woodturning lathe and milling machine, and I'm about to change my second wood lathe and my metal lathe to 3 phase - the speed control is so useful on these. My inverter is on default settings and it just seems to work on whichever motor I connect to it.

Take a look at Drives Direct website - they deal in inverters and often have special offers - they're on ebay as well. Make sure you get one that is rated at a high enough HP for whatever you are likely to get, but of course, higher HP means higher prices. Mine is 2HP and covers everything I need.

K

 

[Bills Elm]

K

Yes I see what you mean about "shaft size, mounting issues, physical size of motor etc"

It's for a bandsaw so speed control isn't so important but what you mention above could be a real hard thing to match up, so I think I'll look for a cheap inverter.

Am I right in thinking that if I have a 2hp motor a 3hp inverter will be ok to use or do I need to match the hp exactly or, can I adjust this in the software of the inverter to match the 2 horse motor?

Bill

 

[porker]

To run a 2HP motor you can use anything 2HP or above, so yes a 3HP drive could be used to drive a 2HP motor. I have a 1HP VFD running a 1/8 HP milling machine table motor because I have the same model running the head motor but the cost difference for a lower rated unit was negligible. The step up in cost from a 2HP to a 3HP might be quite a bit though.

Because VFDs can soft start they can minimise the inrush current.

 

[Bills Elm]

To run a 2HP motor you can use anything 2HP or above, so yes a 3HP drive could be used to drive a 2HP motor. I have a 1HP VFD running a 1/8 HP milling machine table motor because I have the same model running the head motor but the cost difference for a lower rated unit was negligible. The step up in cost from a 2HP to a 3HP might be quite a bit though.

Because VFDs can soft start they can minimise the inrush current.

Thanks Porker so if I owned a 10hp inverter I could run a 1/2hp motor on it, yeah?

Bill

 

[graduate_owner]

Hi Bill,
you could theoretically run a small motor on a 10HP inverter, but you would lose the benefit of motor protection which inverters also provide. There would be no problem under normal conditions, but if a fault occurred such as the motor overheating then I doubt a 10HP inverter would react, whereas a smaller inverter would detect the fault and shut down. Again, this may be something that the inverter parameters can be set up to deal with but I'm not sure, and it would depend on the inverter. Basically under normal conditions you should be fine with an over-specified inverter, and of course, if you ever need to use it for a larger HP machine, then you would have that option without having to upgrade inverters. Just make sure the inverter gets plenty of cooling - it will probably have a fan for forced air cooling, and probably a thermal cut out, but if the fan starts to act up (it has moving parts - they wear out) then make sure you replace it. I disconnected the fan on mine as it was acting up, and replaced it with a cooling fan from an old computer psu fan, fed from a separate 12v supply. No problems to date - it was a bigger fan anyway.

K

 

[porker]

As graduate_owner says...

My Teco VFD has configurable parameters that allow for overcurrent protection etc. but out of the box they are set with numbers reasonable for the quoted capacity.

I am guessing you know this but I will state it anyway. VFD outputs are normally connected directly to the motor that they are driving (i.e. there are no switches or contactors etc. in between the outputs) as they need to "see" a load at all times. Some of them can be connected to supply a machine as a plug in replacement for a three phase supply but this is the exception to the rule. I think these are known as "plug and play" systems. The downside therefore to VFDs (as opposed to rotary or static converters) is that there is normally an element of rewiring required in the machines regarding stop/start buttons, e/stops etc. If you are reasonably electrically minded this is not too difficult.